1. Field of the Invention
The present invention is related to measurement of fatigue caused by vibration and, more particularly, to the measurement of accumulated metal fatigue in the rotor of a turbine.
2. Description of the Related Art
Conventionally, stresses in turbine blades, like other devices, are measured using strain gauges which attach directly to the turbine blade and communicate to monitoring equipment outside the turbine via slip rings or telemetry. An alternative system has undergone development at the Arnold Engineering Development Center of the U.S. Air Force Systems Command. The Arnold system utilizes line-focusing photo detectors which provide non-interference measurement of jet engine turbine rotor blade deflection. The sensors are closely spaced on the periphery of the engine casing with each sensor radially aligned and the group of sensors aligned with the path of the rotor blade tips. Two sensors are used for detecting non-integral vibration, i.e., vibration at frequencies other than multiples of the rotation speed. Four sensors are used for detecting integral vibration, i.e., vibration at a frequency which is a multiple of the rotation speed. The signals produced by these sensors can be processed with reference to a once-per-revolution signal to determine amplitude, frequency and phase of the vibration of the each blade in a row. An additional set of these sensors is required for each row which is monitored.
Using the deflection data provided by the sensors, stress information for all blades in each instrumented row can be determined on-line by using transfer functions to convert the deflection data to stress. These transfer functions can be determined using finite-element analysis and bench-testing of each row of blades before operational testing. While the information provided by the Arnold system is useful, particularly in testing recently serviced turbines, additional information is desirable for an on-line system which is used to monitor a turbine during operation.